Its nowhere near to 15% performance increase for a few reasons.
The memory chips are the same on the 5850 so they overclock to the same speed and same buss, same bandwidth. The 5850 has 2 x 6 pin connectors so it is not power starved which was the case with the 4850 and you can control the voltage with a BIOS flash. So the core can be clocked to the same speed.
A 5850 can be clocked to the same speeds as 5870 the only difference between the GPU's is 10% less Shaders and texture units on the 5850.
a 10% reduction in only shaders and texture units is not going to lead to a 15% performance loss when you consider everything else is the same like Memory bandwidth, memory size, clock speeds and ROP's.
Also The 5000 series is not shader limited it is ROP limited and the 5850 has the same number of ROPs (32). This leads to a 5% performance difference at most between a 5850 and 5870 which are clocked at the same speeds.
So you gain at most 5% performance by spending 50% more on the 5870. True story :wink:
The 5870 six 2GB version might be worth waiting for though as 2GB of Memory will certainly help with eyefinity resolutions.
I agree with you, but the question is what happens when you overclock the 5870?? It goes ahead again, maybe not 15% but when your at the margin of 25 vs 30fps thats the difference between playable and not playable and with no crossfire support for eyefinity in sight yet I felt personally it was a safer bet to get the 5870.
I had crossfire 4850's and upgraded to the 5870 and I was very happy to get away from dual cards and could not justify the move to a 5850 as it probably would have been not as powerful as my current 4850 crossfire.
To tell the truth there have been a few games so far that sit at like 30fps when on high settings, so I think the 4850 would not have made the cut. The 2GB Six maybe would have been worth the wait but who knows how expensive that card will be, and it may require some $$$ for adapters too.
I happen to have 3x U2410 all of them support display port but we do not know if mini DP --> Full DP will be included in the box yet.
I think I will hold out for the 5870X2 and make that the next logical upgrade from a single 5870, I cant even fit another one of them in my case in crossfire (P182) and a P45 mobo like I use drops from 16x to 8x on each slot in crossfire and in high resolutions that has been shown to decrease performance.
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